Add conversation transcript lock contract

[chubes4]

Summary

• Adds an optional ConversationTranscriptLockInterface with TTL-bounded token acquire/release semantics, plus a NullConversationTranscriptLock reference implementation.
• Wires AgentConversationLoop to acquire a transcript lock when a lock primitive and session ID are provided, skip execution on contention, and release after transcript persistence.
• Adds smoke coverage for acquire/release, contention, TTL reacquisition, stale-token release rejection, and loop lock ordering.

Fixes #74.

Tests

• php tests/conversation-transcript-lock-smoke.php
• php tests/conversation-loop-transcript-persister-smoke.php
• php tests/conversation-runner-contracts-smoke.php
• php tests/workspace-scope-smoke.php
• php tests/bootstrap-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the contract, loop integration, focused smoke tests, and running verification; Chris remains responsible for review and merge.